When finding ruby, skip loading rubygems for the speed

[amatsuda]

Maybe this is yet another solution for #35.

Recent versions of ruby loads rubygems by default, which in some cases adds recognizable delay on simple ruby -e call, for instance when having thousands of gems installed in the system.

Here are results of time executing ruby -e with and without --disable-gems option on my machine (Ruby 2.5.0, Mac OSX).
Not very much significant, but still we can see a little bit of improvement.

% time ruby -e 42
ruby -e 42  0.08s user 0.04s system 85% cpu 0.138 total
% time ruby -e 42
ruby -e 42  0.08s user 0.03s system 94% cpu 0.118 total
% time ruby -e 42
ruby -e 42  0.07s user 0.03s system 83% cpu 0.124 total
% time ruby -e 42
ruby -e 42  0.07s user 0.04s system 94% cpu 0.112 total
% time ruby -e 42
ruby -e 42  0.07s user 0.03s system 93% cpu 0.109 total

% time ruby --disable-gems -e 42
ruby --disable-gems -e 42  0.03s user 0.03s system 80% cpu 0.075 total
% time ruby --disable-gems -e 42
ruby --disable-gems -e 42  0.03s user 0.03s system 92% cpu 0.066 total
% time ruby --disable-gems -e 42
ruby --disable-gems -e 42  0.03s user 0.03s system 92% cpu 0.067 total
% time ruby --disable-gems -e 42
ruby --disable-gems -e 42  0.03s user 0.03s system 81% cpu 0.071 total
% time ruby --disable-gems -e 42
ruby --disable-gems -e 42  0.03s user 0.03s system 92% cpu 0.067 total

[damphyr]

Looks good.
I'm going to run it on Windows as well, I expect there the difference to be even greater. Once I've done it I'll merge 👍